What is the probe to bone test, particularly in elderly patients or those with underlying medical conditions, and how is it used to assess wound depth or potential bone fractures?

What is the Probe-to-Bone Test?

The probe-to-bone (PTB) test is a bedside diagnostic procedure where a sterile, blunt metal probe is inserted through a wound to assess whether bone can be palpated, which helps identify underlying osteomyelitis, particularly in diabetic foot ulcers. 1, 2

How to Perform the Test

• Insert a sterile, blunt metal probe through the debrided wound base to measure depth and extent 1, 3
• Palpable bone has a characteristic "stony feel" or hard, gritty sensation when contacted 1
• The test should be performed after adequate debridement of necrotic tissue and callus to ensure accurate assessment 1, 3
• Document whether bone is palpable, along with wound measurements (depth, width, length) and any communication with joint cavities or tendon sheaths 1, 3

Diagnostic Performance

In high-prevalence populations (pretest probability >60%), a positive PTB test makes osteomyelitis highly likely with a positive predictive value of 89-95% and a positive likelihood ratio of 7.2. 2

Test Characteristics:

• Sensitivity: 66-98% (highly sensitive for detecting osteomyelitis when present) 2, 4
• Specificity: 78-91% (good at ruling out disease when negative) 2, 4
• Negative predictive value: 98% (a negative test effectively excludes osteomyelitis) 4
• Positive predictive value: 57-95% (varies significantly based on pretest probability) 2, 4

Clinical Context and When to Use

The American Diabetes Association recommends performing the PTB test at every consultation for chronic foot ulcers to monitor for development of osteomyelitis, especially in patients with ulcers overlying bony prominences. 2

Specific Indications:

• Chronic foot ulcers failing to heal after 6 weeks of adequate off-loading and wound care 2
• Ulcers located over bony prominences in diabetic patients 2
• Any diabetic foot wound where osteomyelitis is clinically suspected 1, 2

Interpreting Results in Clinical Practice

High-Risk Populations (Pretest Probability >60%):

• Positive PTB test: Osteomyelitis is highly likely; proceed with treatment or confirmatory imaging 2
• Negative PTB test: Osteomyelitis is effectively ruled out 4

Lower-Risk Populations (Pretest Probability ~12%):

• Positive PTB test: Only 57% chance of osteomyelitis; requires confirmatory testing 4
• Negative PTB test: Osteomyelitis is effectively ruled out 4

Enhancing Diagnostic Accuracy

Combine PTB results with plain radiography as the initial imaging approach, which improves overall diagnostic accuracy. 2

Additional Diagnostic Strategies:

• Erythrocyte sedimentation rate (ESR) >70 mm/h combined with positive PTB further increases diagnostic certainty 2
• If diagnostic uncertainty remains after PTB and plain radiographs, obtain MRI, which has 89% accuracy for osteomyelitis 2
• Deep wound cultures after debridement correlate well with bone biopsy cultures (73.5% concordance) and can guide antibiotic therapy 5

Critical Pitfalls to Avoid

• Do not perform PTB through infected or non-debrided tissue: Always debride first to remove colonizing bacteria and necrotic material 1, 3
• Do not rely solely on positive PTB in low-prevalence populations: The positive predictive value drops significantly when pretest probability is low 4
• Do not skip the test in chronic ulcers: Perform at every consultation, as osteomyelitis can develop over time 2
• Ensure adequate analgesia: Pain control is essential for thorough examination 3

Beyond Osteomyelitis Detection

The probe also serves multiple other diagnostic purposes during wound assessment:

• Detects foreign bodies within the wound 1
• Identifies soft-tissue abscesses 1
• Determines communication with joint cavities or tendon sheaths 1, 3
• Measures wound depth for baseline healing assessment 1, 3